The present invention relates to a process for preparing a high-Btu content gas by the reaction of a hydrogen-containing pyrolysis gas with a hydrogen-deficient carbonaceous material at an elevated temperature and pressure. It particularly relates to such a process wherein the pyrolysis gas comprises hydrogen as the principal constituent and the balance consists essentially of carbon monoxide, steam and methane such that substantially no additional external source of hydrogen is required for the process.
It has been known for hundreds of years that a combustible gas could be produced from coal by simply heating the coal in a closed vessel. It was not until the 1800's, however, that a manufactured gas industry began. In the latter part of the 1800's, the water-gas process was developed in which steam was reacted with hot coal to produce large quantities of a combustible gas. The combustible gas produced by the water-gas process had to be enriched by carburetting it (adding volatile hydrocarbons) to increase its Btu content to a level at which it was suitable for illumination applications. This technology was inefficient by today's standards. Nonetheless, it accounted for over half of the manufactured gas production in the United States as late as the middle 1900's. Manufactured gas was gradually replaced by natural gas as new reserves were developed and pipelines constructed for its distribution.
In recent years worldwide concern has arisen over the rapid increase in petroleum and natural gas consumption, and the realization that the world's supplies of petroleum and natural gas are limited. Accordingly, considerable effort is being expended to develop alternate sources of energy. One such alternate energy source is synthetic petroleum and synthetic (or substitute) natural gas (SNG) produced by the conversion of coal. The advantages of converting coal to a substitute natural gas (essentially methane) are that the United States has enormous established reserves of coal that can readily be mined, the distribution and end use apparatus for the gas are presently in existence and SNG is a relatively clean burning fuel.
Several coal gasifier systems have been demonstrated on a commercial scale. The most widely known of these is the Lurgi system which is in commercial operation in South Africa. Also in use are the Winkler and Koppers-Totzek systems. All of these systems gasify coal with a mixture of steam and oxygen. A disadvantage of these systems is that a substantial amount of the coal is consumed in generating the heat required for the steam to react with the coal. In addition, the temperatures required for a rapid reaction and significant conversion of the coal are relatively high and do not favor methane formation. Thus, the gaseous products produced must be further processed if the desired product is methane.
It also has been proposed to react coal with hydrogen at elevated temperatures to produce a substantially high yield of methane directly. This approach, however, requires a separate reactor for the production of the large amounts of hydrogen consumed in the process, which adds considerably to the complexity of the process and the expense of the methane produced. Thus, various processes have been proposed to obviate some of these disadvantages and provide a means for the production of methane from coal which is both economical and energy efficient.
U.S. Pat. No. 4,162,959 describes a process for the production of hydrogenated hydrocarbons. A solid carbonaceous material is pyrolyzed in the presence of a particulate source of heat to yield a particulate carbon-containing residue and volatilized hydrocarbons, while simultaneously the volatilized hydrocarbons are hydrogenated. The particulate source of heat is formed by oxidizing carbon in the solid residue to heat the particles. Hydrogen for the process is obtained by reacting at least a portion of the hot particulate carbon-containing residue of pyrolysis with steam prior to feeding the particulate residue to the pyrolysis reaction zone. Thus, this process utilizes three reaction zones; namely, an oxidation zone, a hydrogen generation (conversion) zone, and a pyrolysis zone.
U.S. Pat. No. 4,183,733 describes a hydro gas plant comprising the combination of a gasifier into which raw coal is introduced for hydrogasification that results in the production of methane; a furnace for cracking the methane into hydrogen and carbon monoxide; and means for utilizing the combustion heat of the residual coke obtained during the operation of the gasifier. Thus this process obtains its hydrogen for hydrogasification by cracking a portion of the methane product.
U.S. Pat. No. 4,217,201 describes an integrated coal cleaning, liquefaction and gasification process. In the disclosed process, coal is finely ground and cleaned so as to preferentially remove denser ash-containing particles along with some coal. The resulting clean coal portion having a reduced ash content is then fed to a coal hydrogenation system for the production of desirable hydrocarbon gases and liquid products. The remaining ash-enriched coal portion is gasified in a separate reactor to produce a synthesis gas which is then shift converted with steam and purified to produce high purity hydrogen for the coal hydrogenation system. Thus this process requires two separate reactors--one for hydrogenation and another to produce a synthesis gas for conversion to hydrogen for use in the process.
Another process for the production of substitute natural gas from coal utilizing catalytic gasification is described by J. E. Gallagher, Jr. and H. A. Marshall of Exxon Research and Engineering Company in "Production of SNG from Illinois Coal Via Catalytic Gasification," A.I.Ch.E. Symposium on Reaction Engineering in Processing Solid Fossil Fuels, November 1978, Miami Beach, Fla. In the disclosed process, dried coal is coated with potassium hydroxide catalyst. The coated coal is then reacted in a fluidized bed with a preheated mixture of steam, recycle hydrogen and carbon monoxide to produce a product gas substantially free of any heavy hydrocarbons. Any acid gases present in the product gas are removed, and the remaining gas, consisting essentially of only hydrogen, carbon monoxide and methane, is sent to a cryogenic distillation system. The methane is separated, and the CO and H.sub.2 are mixed with gasification steam and recycled to the gasifier. In the presence of the catalyst, the carbon monoxide and hydrogen react to produce methane and generate heat in a sufficient quantity to substantially offset the concurrent endothermic reaction between the steam and coal. The principal disadvantage of this process is that it requires a catalyst and a separate catalyst recovery system.
In spite of the intensive research being done in the area of coal gasification there still exists a need for further improvement in processes for the gasification of coal to produce methane.